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HOMESERVICE MANUALSAUDI1999A8 QUATTROREPAIR AND DIAGNOSIS (SINGLE PAGE)ACCESSORIES & EQUIPMENTWARNING SYSTEMSON BOARD DIAGNOSTIC (OBD) - ABS/EDL/ASRCAN-BUS, FUNCTIONTWO-LINE BUS SYSTEM, CHECKINGTWO CONTROL MODULES ARE COMMUNICATING ACROSS A "TWO-LINE BUS SYSTEM"
1999 Audi A8 Quattro

Two control modules are communicating across a "two-line bus system"

1999 Audi A8 QuattroSECTION Two control modules are communicating across a "two-line bus system"
  • Switch ignition off.
    Fig 1: Schematic - Disconnecting Both Control Modules
    G02727023Courtesy of AUDI OF AMERICA, INC.
  • Disconnect both control modules.
  • Check the bus lines for an open circuit.

    See appropriate System Wiring Diagrams article.

    Fig 2: Schematic - Checking Short Circuit Between Bus Lines
    G02727024Courtesy of AUDI OF AMERICA, INC.
  • Check whether there is a short circuit between the bus lines.

    See appropriate System Wiring Diagrams article.

  • Check the bus lines for a short circuit to B+ or Ground (GND).

If no malfunction is found in the bus lines:

  • As a test, replace one of the control modules. Select a control module that will take the least amount of time, and cost the least amount of money to replace.
  • If communication between the control modules across the bus is still interrupted, replace the second control module.
Fig 3: Schematic - Checking Bus Lines For A Short Circuit To B+ Or Ground (GND)
G02727025Courtesy of AUDI OF AMERICA, INC.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.